This invention relates to a power amplifier for receiving an input informational signal and providing an amplified output informational signal which is substantially free of distortion.
Hi-fidelity circuits which respond to electrical audio signals such as commonly found in hi-fi and stereo phonograph systems, tape playing systems and possibly in radio systems or the like commonly utilize transistorized circuit configurations for efficient power transfer and signal amplification between a pre-amplifier operating in conjunction with a magnetic phono cartridge, tape head or other pick-up device and one or more loud speakers. It is essential for hi-fidelity reproduction of electrical audio signals to provide a wide frequency response which is particularly critical at the lower end of the audio frequency range and at the same time provide little or no distortion between the input and output signals.
Non-linear distortion has frequently existed in prior audio power amplifiers as evidenced by new frequencies appearing in the output which were not present in the input. Inter-modulation distortion also creates problems such as when the input signal contains more than one frequency and the output contains frequencies equal to the sums and differences of the input frequencies and of their harmonics. It is desirable to eliminate or substantially reduce such non-linear and inter-modulation distortion in order to provide desirable hi-fidelity signal reproduction which is particularly important for pleasant music listening in commercial sound systems.
Many electrical audio power amplifiers have utilized transistor circuits employing negative feedback to provide a wide frequency response and some distortion elimination. Some systems have employed voltage feedback over one or several transistor stages as supplied from a collector circuit of an output power transistor while other systems have employed a current feedback as supplied from an emitter circuit of an output power transistor, with either of such signals being fedback to a base circuit of an input transistor also coupled to receive the input informational signal from the pre-amplifier stage. Such systems had to be carefully designed to prevent high frequency instability such as might be caused when the phase shift through the transistor amplifier was sufficient to change the feedback from negative to positive magnitudes. Frequently, the frequency response of the feedback loop was limited to stabilize the circuit. In addition, the amount of feedback that could be applied to some audio power transistors was limited because of the poor frequency response in many of the common emitter and common collector connections.
Some audio power amplifiers have employed a pair of output power transistors which are directly coupled to drive one or more speakers thereby eliminating transformer coupling circuits. Such direct coupled audio power amplifying output transistors have frequently been connected in single ended class B push-pull arrangements for providing an excellent low frequency response.
Some audio power amplifiers have employed an input differential amplifier which provided a pair of amplifying transistors having emitter circuits connected in common configuration to a constant current source with the base circuit of a first transistor connected to receive the input informational signal from the pre-amplifier stage while the collector circuit provided an intermediate amplified signal responsive to the input informational signal. The second transistor stage of the input differential amplifier provided a base circuit which was connected to a current feedback circuit which supplied a compensating control signal in response to the amplified output informational signal for controlling the current flow from the constant current source through the second transistor amplifying stage of the input differential amplifier. The variance of current flow through the second transistor also correspondingly compensated the current conduction through the first transistor and provided partial distortion elimination in the intermediate amplified signal. While satisfactorily eliminating some distortion in the audio power amplifier, such a fedback compensating control signal had to be scaled to a substantially low magnitude with respect to the input informational signal in order to provide a differential output at the collector circuit of the first transistor stage. Simply increasing the magnitude of the feedback compensating control signal tended to null the differential intermediate output signal rendering the power amplifier inoperable or at least tended to reduce the amplification efficiency to a point where the power amplifier would be commercially undesirable. Thus, such compensating feedback to an input differential amplifier stage could only provide partial compensation while objectionable non-linear and inter-modulation distortion remained.